MVG / VMVG Justification Analysis

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Why do you need to verify your CMM or Vision Machine?

 

Machine Calibrations only provide accuracy data for a given moment in time

All parts checked between calibrations become suspect if machine is out of tolerance at calibration

With daily verifications, the maximum amount of suspect parts can be reduced to one days production

Completely cover procedures governing control and detection of non-conforming parts

Sources of error on CMM and Vision Machine

Other Sources

 

 Machine Repeatability

 

 Sensor Repeatability

 

 Sensor Accuracy

 

 Temperature

What is checked during a calibration (CMM) ?

 

Volumetric performance of the machine - An artifact called a ball bar is checked in 20 different positions on the machine. The length variation must not exceed the OEM spec. This test gives insight into machine pitch, yaw, roll and squareness.

Linear accuracy of the machine - An artifact of known length is measured in each axis. The length variation must not exceed the OEM spec.

Repeatability of the machine - An artifact is measured 10 times in the same position on the machine. The position error must not exceed the manufacturer spec. Note this test indirectly includes the Probe Repeatability.

What is machine verification ?

Machine verification is the periodic inspection of an artifact with data relation back to a known standard to ensure the machine is still within performance specifications.

The ideal verification system would have the following characteristics

1) Have a 5 minute or less runtime
2) Require no special tips to run (use tip 0)
3) Be an uncalibrated artifact used in conjunction with calibrations
4) Encompass the volume of the machine where parts are run
5) Report back as many error parameters as possible
6) Automatically alert when tolerances are exceeded

Extract

 

Advantages in Use of a "wellness" type gage (artifact) for CMM equipment

 

 

This document was created to investigate the benefits of using a gage (artifact) that can accomplish the following goals related to CMM maintenance and calibration costs.

1) Keeping questionable product from reaching the customer

2) Assure CMM is checking properly on a daily basis

3) Use of artifact in keeping Calibration and/or Certification costs to a minimum

4) Discovery of further benefits with the use of the artifact

5) Identifying the other requirements or tools that should be available in conjunction with this artifact

Our first goal for using a CMM is to keep the customer from getting product that is questionable. The customer in this case may mean the assembly line, or the next operation for the subject part. Either way, relying on the last Calibration, which could be up to a year old, may not be the best plan. If you were to document the results from a part and the program used to measure it immediately after a Calibration, and then repeated this procedure at the start of the day, you could accomplish this task. This may however mask some problems with the CMM, and manually comparing the results each time would become labor intensive. The system then becomes prone to human error and can eventually cost more than using a specific artifact to check the machine. An object or artifact combined with the proper software can quickly verify changes from 'yesterday' or the 'last shift', preventing questionable parts from reaching the customer. It is easier and less expensive to quarantine parts from a single shift than to recall up to the last 6 months or more of production.

The need to keep questionable parts from reaching our customers by using a quick check artifact would also ensure that the CMM is checking as we expect. We can verify this using hard data to ensure the CMM is checking properly.

How does having this artifact help us save money on Certification and Calibrations? Ideally, software should have tracking capabilities built-in to let us know the trends of the machine. For example if the machine only needs to be calibrated annually, we should know this by having the software track changes for us. Some machines may require calibration more often, and this will still save us money by keeping bad or questionable parts from reaching the customers. Keep in mind that this artifact itself does not necessarily need to be calibrated, serving to reduce overall costs.

It may be noteworthy to explain the difference between CMM Calibration versus Certification. These terms should not be used interchangeably. Calibration means the personnel servicing the machine should have the capability to bringing it back to OEM specifications. Certification is verifying what the machine is capable of as far as accuracy, and is not a correction to meet OEM specification.

Now that we have an idea of what the "wellness" gage (artifact) can do on a regular basis, we might ask the question of what other benefits can be obtained from such a system. Having this system available you could check different areas of a larger machine to see if geometry problems are present. You could quickly compare the results of one CMM to another as a simple correlation device. Although CMMs normally do not correlate exactly, they should be reasonably close (within OEM specifications). You could compare different "brands" of CMMs and their different cost of ownership (does brand X have a problem with keeping good linear or squareness results when compared to brand Y?). Does Brand X need calibration more frequently the Brand Y? You may determine that you would like to have certain CMMs calibrated and have a correlation (a comparison of measurement results) done at the same time.

When considering such an item for "wellness" checks, comprehensive software to track any changes should be considered. Preferably, this software will be part of a "package" along with an artifact that was designed to get the most data from such a program. This check should include at a minimum a quick check for squares and linear data (the heart of CMM accuracy).

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